1. Field
The present invention relates to the use of oxidising agents in adhesive compositions suitable for use in polymer to substrate, such as elastomer to substrate, for example polymer-to-metal, such as elastomer-to-metal including rubber-to-metal bonding applications. The oxidising agents transform a latent cross linking agent into a species capable of actively crosslinking elastomers comprising diene and or allylic functional groups and are suitable for use in the above bonding applications and in particular in rubber to metal bonding applications.
2. Brief Description of Related Technology
Polymer to metal and in particular rubber to metal bonding has been practised for many years. There are many applications for formulations which achieve polymer or rubber to metal bonding. Rubber to metal bonding is widely used to bond different metals to a natural or synthetic rubber. Polymer to metal bonding is carried out for many reasons.
One aspect of rubber to metal bonding is to combine the structural strength of the metal with the elastomeric properties of the rubber. Accordingly, metal and polymers such as rubber, in particular, are often bonded to each other for impact absorption applications, such as bearings, wheels, shock absorbers, moving arms, etc. Such components can be utilised on a very small scale, for example in PC components or on a very large scale for example in constructions such as bridges and buildings. Noise reduction may also be achieved by utilising metal to rubber bonding. It is accepted that tremendous forces can be experienced by any component which comprises metal and rubber bonded together. Accordingly, it is desirable to provide metal to rubber bonding that can withstand significant forces such as compressive or extensive forces, including shocks, without having the metal or the rubber separate from each other. There are many other applications where rubber to metal bonding is desirable, for example in tyre production where internal wire reinforcements for the tyre are bonded to the rubber of the tyre.
Generally, an adhesive formulation is provided to bond a selected rubber to a selected metal substrate. Traditional rubber-to-metal bonding technology consists of a primer layer and also an adhesive layer, forming an overall 2-coat system. The primer system generally consists of solutions or suspensions of chlorinated rubber and phenolic resins containing reactive groups, and also pigments such as titanium dioxide, zinc oxide, carbon black, etc. The primer is generally applied as a thin layer onto a treated (cleaned) surface of a metallic component such as a treated steel component for example a component that has been grit blasted or chemically treated.
The adhesive layer can consist of a large range of elastomeric materials such as rubber materials and cross-linkers. These include, but are not restricted to, chlorinated and bromochlorinated rubbers, aromatic dinitrosobenzene compounds and bismaleimide as cross-linkers, xylene, perchloroethylene and ethylbenzene as solvents, and also some inorganic lead or zinc salt.
The most common cross-linking agents that have been employed in rubber-to-metal bonding technology are aromatic nitroso compounds, such as p-dinitrosobenzene. The mechanism of cross-linking to an alkene group within a rubber, such as a diene or allylic moiety, is by means of a pericyclic reaction.
In the many fields where rubber to metal bonding is employed there are numerous opportunities to address several types of problems, including bond strength and durability, sample preparation, ease of application of the adhesive, single coat versus two-coat systems, reduced toxicity and improved properties for the environment, to name a few.
Dinitroso compounds are ordinarily used in rubber bonding formulations. The high toxicity of these dinitroso compounds, in particular, poses serious handling and safety problems. In addition dinitroso compounds can exhibit fuming at elevated temperatures which aggravates the problem of mold fouling.
Different formulations for rubber to metal bonding are the subject of many patent publications. Lord Corporation has conducted studies in the area of use of oxime and dioxime materials. For example as set out in European Patent Document No. EP0287190, an adduct of any aromatic dioxime (quinone dioxime is preferred) and a polyisocyanate (toluene diisocyanate is preferred) is formed in a ratio of greater then 2:1, and rubber to metal bonds formed.
U.S. Pat. No. 4,581,092 discloses an adhesive system that includes a butyl rubber, a polyisocyanate compound, and at least one of a nitroso compound and an oxime compound, with the oxime compound requiring the additional presence of an oxidizing agent, including Pb3O4, PbO2, MnO and V2O5. The oxidizing agent is present at a level of 2 to 3 parts per part of aryl dioxime compound.
U.S. Pat. No. 3,824,217 discloses combining an oxime compound with an excess of a polyisocyanate compound, so that all oxime groups react with isocyanate. The resulting compound could be used in compositions for bonding rubbers to primed metal substrates. U.S. Pat. No. 3,859,258 also reports an oxime-isocyanate product for use in rubber to metal technology.
U.S. Pat. No. 4,994,519 is directed to a quinone dioxime system that has rubber to metal bonding performance without the use of an oxidant, when present with chlorinated rubber, brominated polybutadiene, carbon black and from 5 to 25% by weight of ground sulfur. The use of free sulfur however is not widely accepted within the industry.
U.S. Pat. No. 4,031,120 describes a composition comprising an isocyanate functional organosilane, in combination with a polyisocyanate and an aromatic nitroso compound. The resulting system is described as a one-coat adhesive for bonding a variety of elastomeric materials to metals and other substrates. In this patent an isocyanate is reacted with a functional alkoxy silane such as amino, hydroxy, or mercapto.
Notwithstanding the state of the art it would be desirable to provide alternative elastomer bonding compositions addressing the issues raised above, such that improved consumer choice results.